1. Field of the Invention.
The present invention relates to high volume virtual impactors for collection of samples of particles carried in the air.
2. Description of the Prior Art.
Virtual impactors have been known in the prior art. Virtual impactors include an input nozzle aligned with a receiving tube. Air carrying particles to be collected is drawn through the input nozzle and separated into major flow and minor flow. The major flow is diverted from entering the receiving tube while the minor flow passes into the receiving tube along with the large particles, which are carried into the receiver tube in inertia. The air is then filtered for analysis of the particles.
U.S. Pat. No. 4,301,002 illustrates such a device, and shows a single virtual impactor apparatus mounted in a frame for handling relatively low volume flows. The receiving tube in Patent '002 has its end closed with a large particle filter, while the major flow is diverted from the receiving tube and passes through a separate small particle filter. A single air pump or blower is used for providing the separated major and minor flows needed.
The virtual impactor shown in U.S. Pat. No. 4,301,002 shows some of the problems associated with this type of collector, as well as indicating advantages, but it provides no solution to the problem of handling relatively large flows and obtaining reliable samples.
U.S. Pat. No. 4,452,068 also shows a device for trapping aerosols, utilizing flow separation principles, but having a grooved impaction surface for receiving the inlet air and providing an inertial trap for particles coming from the inlet.
Impactor devices for collecting airborne particles are shown in U.S. Pat. Nos. 4,133,202, issued to Marple on Jan. 9, 1979, and 4,321,822, issued to Marple et al. on May 30, 1982.
In the article entitled "Virtual Impactors: A Theoretical Study", (by Marple et al., Environmental Science & Technology, 1980, 14, 976), the characteristics of virtual impactors are analyzed as to operation, effect on collection using different dimensions of nozzles, and other constructional factors. Also, in Aerosol Science and Technology 2:455-464 (1983) Solomon et al. published a study on a High-Volume Dichotomous Virtual Impactor for the Fractionation and Collection of Particles According to Aerodynamic Size. This study used a single acceleration nozzle for the virtual impactor.
A multi-stage virtual impactor, which used a single flow but had more than one stage of separation is disclosed and discussed in a paper entitled Design of a Multi-Stage Virtual Impactor by Novick et al. and published in the work Aerosols, Liu, Pui, and Fissan, editors, Elsevier Science Publishing Co., Inc. (1984).
Up to now, however, high volume virtual impactors that preserve the integrity of the samples collected, avoid excessive impaction of the particles on housing surfaces, and permit collection of aerosol samples quickly for practical application in the field have not been advanced. The present device provides such an impactor.